1. Field of the Invention
This invention relates to an optical recording medium of phase change type and a method for preparing such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. Optical recording media of the phase change type are of great interest since they can be directly overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used (Ge,Sb)-Te based system including Ge-Te, Sb-Te, or Ge-Sb-Te as the main component, which provides a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
In the conventional optical recording medium of phase change type, the recording layer has been formed by such means as vacuum deposition apparatus, and the recording layer immediately after its formation is in non-crystalline state. When the disc having such recording layer is utilized for a rewritable medium, crystallization of the recording layer is generally required and such crystallization is accomplished by a process called initialization.
Various processes have been proposed for the initialization. Typical processes are the process wherein the substrate is heated to crystallization temperature of the recording layer after the deposition of the recording layer (JP-A 3131/1990); the process called "solid phase initialization" wherein the recording layer is crystallized after its deposition by laser beam irradiation (JP-A 366424/1992, 201734/1990, and 76027/1991); a process wherein the substrate having the recording layer deposited thereon is irradiated with a flash light to utilize optical properties of the chalcogen compounds for pseudo-crystallization of the recording layer by photodarkening (JP-A 281219/1992); a process wherein the recording layer is crystallized by means of RF induction heating; a process wherein the substrate is heated simultaneously with the deposition of the recording layer for crystallization (JP-A 98847/1990); a process wherein a dielectric layer is formed as the first layer, and the recording layer is formed on the first layer and heated for crystallization, and the second dielectric layer is formed on the crystallized recording layer (JP-A 5246/1990).
Initialization by laser beam irradiation, however, is a time-consuming process and this process also suffers from insufficient productivity. On the other hand, the process involving the heating of the entire medium prohibited use of inexpensive resin substrates, since the heating during the initialization resulted in deformation of the resin substrate to result in tracking problems. Use of flash light required repeated irradiation to accomplish the crystallization, and productivity was also insufficient.
In view of such situation, an apparatus called "bulk eraser" is currently used for the initialization in commercial scale production. A bulk eraser is an apparatus which is capable of irradiating a high power gas laser or semiconductor laser beam without tight focusing so that multiple tracks can be crystallized at once. Use of such bulk eraser enables localized heating of the recording layer, and temperature elevation of the substrate is thus avoided to enable the use of a resin substrate of low heat resistance.
Initialization with a bulk eraser of a 12 cm optical recording disc, however, usually takes about several ten seconds to several minutes and the process of initialization has been the rate-determining step in the production of the optical recording disc.
In order to eliminate the step of initialization which had been necessary in the production of the phase change type recording medium, the inventors of the present invention proposed in the U.S. Pat. No. 5,627,012 formation of the In-Ag-Te-Sb-based recording layer in separate steps of sputtering Sb+In and sputtering Ag+Te; or alternatively, in separate steps of sputtering Sb, sputtering In, and sputtering Ag+Te. The recording layer formed by such step is at least partially crystallized. The recording layer formed by such process exhibits change in reflectance as in the case of initialization by bulk eraser as described above after full diffusion and mixing of the elements in the layer by repeated recording.
The U.S. Pat. No. 5,627,012, supra, however, is silent about the method of reducing the initialization energy for the phase change-type (Ge,Sb)-Te-based recording layer. In addition, in the optical recording medium described in the U.S. Pat. No. 5,627,012, the erasability is unstable during the first several overwriting operations immediately after the formation of the recording layer as in the case of conventional recording medium of phase change type. More specifically, reflectance of the region crystallized during the formation of the recording layer is different from the reflectance of the region crystallized in the overwriting, and the reflectance are not stabilized until whole recording layer are overwritten. In the mark edge recording used in rewritable digital video discs (DVD-RAM) and the like, such variation in the reflectance may be erroneously recognized as mark edge.
JP-A 106647/96 discloses a phase change type recording medium having disposed thereon a recording layer of an AgInSbTe-based artificial superlattice film wherein AgSbTe.sub.2 film and In-Sb film are disposed one on another, or AgSbTe.sub.2 film, In film and Sb film are disposed one on another. One merit of JP-A 106647/96 is reduction in initialization energy of the whole recording layer owing to the use of the crystallized AgSbTe.sub.2 film.
The JP-A 106647/96, however, is silent about the method of reducing the initialization energy in the phase change-type (Ge,Sb)-Te-based recording layer.